Re: [linux-pm] [PATCH] PERF(kernel): Cleanup power events V2

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On Tue, 26 Oct 2010, Mathieu Desnoyers wrote:

> * Peter Zijlstra (peterz@xxxxxxxxxxxxx) wrote:
> > On Tue, 2010-10-26 at 11:56 -0500, Pierre Tardy wrote:
> > > 
> > > +       trace_runtime_pm_usage(dev, atomic_read(&dev->power.usage_count)+1);
> > >         atomic_inc(&dev->power.usage_count); 
> > 
> > That's terribly racy..
> 
> Looking at the original code, it looks racy even without considering the
> tracepoint:
> 
> int __pm_runtime_get(struct device *dev, bool sync)
>  {
>         int retval;
> 
> +       trace_runtime_pm_usage(dev, atomic_read(&dev->power.usage_count)+1);
>         atomic_inc(&dev->power.usage_count);
>         retval = sync ? pm_runtime_resume(dev) : pm_request_resume(dev);
> 
> There is no implied memory barrier after "atomic_inc". So either all these
> inc/dec are protected with mutexes or spinlocks, in which case one might wonder
> why atomic operations are used at all, or it's a racy mess. (I vote for the
> second option)

I don't understand.  What's the problem?  The inc/dec are atomic 
because they are not protected by spinlocks, but everything else is 
(aside from the tracepoint, which is new).

> kref should certainly be used there.

What for?

Alan Stern

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